Normalize header colors in Remarkup to {$bluetext}

Summary: Increases spacing around Sections and normalizes all header colors to {$bluetext}. Will follow up with T7444.

Test Plan: Test Changelog copied to my local install.

Reviewers: epriestley, btrahan

Reviewed By: btrahan

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D12583